The miniaturization of electronic equipments results in stronger demand to provide smaller and thinner motors for use therein. As such a motor, Japanese Utility Model Laid-Open No. 58-172345, for instance, discloses a flat brushless motor comprising stator coils and a rotor having a permanent magnet opposing thereto, stator coils being constituted by laminating pluralities of sheet coils each formed by a spiral conductor pattern formed on an insulating substrate.
In this flat brushless motor, as shown in FIGS. 14 to 16, motor magnetic poles are formed by 3-phase, 8-pole sheet coils 50 fixed to a stator yoke such that they are opposing to a magnetic pole surface of the rotor magnet 100. A single-phase coil is formed by one 8-pole flat coil, and adjacent coil poles 200 are series-connected with opposite polarities facing each other. Pluralities of sheet coils 50 are concentrically laminated, each coil 50 being held by a coil holder 125 such as a yoke, a print circuit board (PCB), etc., and ends of each coil 50 being connected to a pattern surface of a print circuit board (PCB) 120 by solder, etc. Each sheet coil 50 is a toroidal conductor pattern formed on a thin insulating sheet 210 by etching, plating, etc., which is provided with coil poles 200.
JP 64-59902 A discloses a laminate coil for a brushless motor, which is obtained by forming as coil sheets coil conductor patterns on green sheets formed by ceramic powder, for instance, by a screen printing method, etc., laminating pluralities of coil sheets, providing connection between the coil conductor patterns via through-holes, and integrally burning the coil conductor patterns with the green sheets.
However, such conventional coils suffer from the following problems. In the coil disclosed by Japanese Utility Model Laid-Open No. 58-172345, because three-phase coils are obtained by forming toroidal pattern conductors for coil poles on thin insulating sheets by etching, plating, etc., and disposing them with displacement of a predetermined angle in a circumferential direction, (1) laminated coil layers should be bonded by uniform adhesive layers; (2) positional displacement between the bonded coil layers causes short-circuiting, etc. between the coil poles, resulting in the likelihood of uneven rotation and torque ripple; (3) sheets of polyimides, polyesters, etc. used as insulating sheets have limited mechanical strength, generally necessitating the insulating sheets to be disposed on the PCB or the yoke, thereby making further miniaturization and thinning difficult.
In the laminate coil of JP 64-59902 A, in which green sheet substrates having external terminal electrodes are laminated with pluralities of coil sheets, circular portions, on which coils are formed, and rectangular portions, on which external terminal electrodes are formed, are different in thickness, so that the laminate coil cannot be produced at a high productivity. In addition, to provide the external terminal electrodes with improved mechanical strength, green sheet substrates having no coil conductor patterns formed should be thick, resulting in a thicker laminate coil. In addition, the existence of the external terminal electrodes enlarges the area of the laminate coil, resulting in a larger brushless motor.